Air-driven hydraulic pump with pressure control

ABSTRACT

A hydraulically driven pump with driving pressure controlled within a predetermined range. The hydraulically driven pump has a driving-fluid port fluidly coupled to a compressed air source and ambient, a driven-fluid inlet port fluidly connected to a tank, and a driven-fluid outlet port. A pressure sensor is used for providing sensing values indicative to the driving pressure in the hydraulically driven pump, and the driving pressure is controlled in closed loop by releasing and filling air through the driving-fluid port. The hydraulically driven pump has a suction stroke, in which driven fluid is refilled into the pump, and a pressing stroke, in which driven fluid is pressed out. A hydraulic buffer is used to provide driving pressure during a suction stroke and two hydraulically driven pumps can work alternately in providing continuous pressure control.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

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REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX

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FIELD OF THE INVENTION

This present application claims priority from U.S. provisional application No. 61/520,630 having the same title as the present invention and filed on Jun. 13, 2011.

This invention relates to pumps, and more particularly, to hydraulic injection pumps used in injecting fluid into a vessel, the pressure inside which is controlled within a predetermined range.

BACKGROUND OF THE INVENTION

Air-driven hydraulic pumps use compressed air to drive reciprocating actions in delivering pressurized liquid. With a stepped piston having its large diameter side contacting compressed air in an air cylinder and small diameter side driving liquid in a high pressure injection cylinder, an air-driven hydraulic pump is able to provide high driving pressure, which is multiple times of compressed air pressure, and the multiplication ratio is determined by the ratio of the large diameter to the small diameter. To complete a reciprocation cycle, which includes a suction stroke and a pressing stroke, it needs to control the air pressure inside the air cylinder by filling and releasing compressed air. Normally, the pressure control is realized by using relay valves that use sealed air and switches to fill and release the sealed air in controlling the relay valves (U.S. Pat. Nos. 3,963,383, 4,645,431, and 6,386,841). Therefore, the reciprocating rate is determined by air pressure, air filling and releasing rate, and switch position. As a result, fluctuations in compressed air supply pressure affect both reciprocating rate and driving pressure. Also, in the pressing stroke, compressed air expands and results in pressure drop. The pressure change in compressed air is then amplified by the pump and causes larger change in driving pressure.

To accurately control the driving pressure, a primary object of the present invention is to provide controls means to adjust the driving pressure, thereby with a closed-loop control, the driving pressure can be controlled within a predetermined range.

A further object is to replace the relay valve using the controls means set forth in the foregoing object to provide a simplified pump structure.

A further object is to provide controls means to avoid the effects of the suction stroke in controlling driving pressure.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, a hydraulically actuated pumping apparatus with driving pressure controlled within a predetermined range is provided.

According to one embodiment of the invention, an air-driven hydraulic injection pump is provided that has a pressure multiplication ratio of 1.0, however, has no piston device inside. The stroke control and pressure control are accomplished by energizing and de-energizing two solenoid valves to control air pressure inside the pump according to pressure sensing values obtained from a pressure sensor.

According to another embodiment of the invention, an air-driven hydraulic injection pump is provided that has a pressure multiplication ratio higher than 1.0. This pump has a piston inside and strokes and pressure are controlled by energizing and de-energizing two solenoid valves to release and fill compressed air according to pressure sensing values obtained from a pressure sensor.

According to another embodiment of the invention, a hydraulic buffer is provided with an air-driven hydraulic injection pump. The hydraulic buffer decreases pressure drops associated with suction strokes in which compressed air is released from the pump.

According to another embodiment of the invention, a hydraulically driven pump system including two air-driven hydraulic injection pumps are provided. These two pumps work alternately to control driving pressure within a predetermined range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a depicts an air-driven hydraulic pump system with two two-way solenoid valves and a pump without piston;

FIG. 1 b depicts an air-driven hydraulic pump system with a two-way solenoid valve, a three-way solenoid valve, and a pump without piston;

FIG. 2 is a flow chart of a control algorithm for controlling strokes and pressure;

FIG. 3 depicts a cross sectional elevation view of a hydraulic pump housing used with the same stroke and pressure controls in a system as shown in FIG. 1;

FIG. 4 depicts a cross sectional elevation view of a hydraulic pump housing and a hydraulic buffer that is to decrease pressure drops associated with suction strokes;

FIG. 5 illustrates a timing chart of pressure changes in systems with and without a hydraulic buffer;

FIG. 6 depicts a system with two hydraulic pumps working together to control driving pressure within a predetermined range;

FIG. 7 is timing chart of control mode changes in a system shown in FIG. 6

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 a, a pump 100 includes a gas port 101, a liquid inlet port 105, a liquid outlet port 102, and a pressure sensor 103. Through a liquid passage 132, the liquid inlet port 105 is fluidly connected to a port 131 of a liquid tank 130, which contains a fluid 133. Inside the inlet port 105, a check valve 106 only allows liquid to flow from the liquid tank 130 to the pump 100. Fluid in the pump 100 flows out through the liquid outlet port 102, which has a check valve 104 included, while pressure inside the pump is measured by the pressure sensor 103 and the sensing value is sent to a controller 110. The gas port 101 of the pump 100 is fluidly coupled to the outlet of a two-way solenoid valve 126 through a Tee connector 127 and an air passage 125, and the inlet of the solenoid valve 126 is connected to a compressed air supply (not shown in FIG. 1 a). The Tee connector 127 is also fluidly connected to the inlet of another two way solenoid valve 122 through an air passage 121. To lower down the noise when releasing air, an optional muffler 124 can be connected to the outlet of the solenoid valve 122 through an air passage 123. Both of the solenoid valves 122 and 126 are electrically connected to the controller 110. At normal states, i.e., before the solenoid valves 122 and 126 are energized, the compressed air is disconnected to the gas port 101 and the gas port 101 is fluidly connected to ambient through the solenoid valve 122.

Stroke control and pressure control for the pump 100 are accomplished by using the combination of controls to the solenoid valves 122 and 126. The controls to the two valves have four modes shown in the following table:

TABLE 1 Mode Status of the Status of the number valve 126 valve 122 Actions 0 Not energized Not energized Releasing air from pump 1 Not energized Energized Keeping air in pump 2 Energized Not energized Leaking compressed air 3 Energized Energized Filling air to pump

In Mode 0, both of the solenoid valves 122 and 126 are not energized, and the pump is releasing air to ambient. In Mode 1, since solenoid valve 122 is energized, the pump is disconnected from ambient. At the same time, the solenoid valve 126 is not energized. Therefore, in this mode, the air is trapped in the pump. Mode 2 is a special mode needs to be avoided, since in this mode the compressed air is directly released into ambient. Mode 3 is an aspiration mode. In this mode, the solenoid valve 122 disconnects the pump from ambient, while the solenoid valve 126 fluidly connects the pump the compressed air supply.

The pumping control starts with a suction stroke. When fluid in the pump reaches a certain level, a pressing stroke is triggered and driving pressure is controlled in a range commanded by the user. The pump goes back to suction stroke when a refill event is triggered.

In a suction stroke, Mode 0 is triggered. As mentioned above, in Mode 0, the pump releases air to ambient. After the air pressure inside the pump drops, under gravity or the pressure inside the fluid tank 130, the fluid 133 flows through the port 131, the passage 132, and the check valve 106 inside the port 105 into the pump. In the suction stroke, no fluid flows out of the pump.

After a suction stroke, the controller enters Mode 3, in which compressed air flows into the pump 100 through the solenoid valve 126, the passage 125, the Tee connector 127, and the gas port 101. Under the air pressure, fluid in the pump is able to flow out through the port 102.

Sensing values obtained from the pressure sensor 103 are used in controlling strokes and liquid driving pressure. One embodiment of a control algorithm is realized with a service routine running periodically in the controller 110 for a timer based interrupt. As depicted in FIG. 2, in this routine, firstly a suction-stroke trigger flag is examined, if a suction stroke is triggered, then the controller goes to Mode 0 to release air from the pump 100. Then the status of the suction stroke is checked. If the suction stroke is completed, then the routine ends after a pressing-stroke trigger flag is set and the suction-stroke trigger flag is reset, otherwise, the time in the suction mode is checked in a step 201. If it is too long, then a fault is set in a step 202, and the routine ends. Referring back to the examination of the suction-stroke trigger, if a suction stroke is not triggered, then the pressing-stroke trigger flag is examined. If a pressing stroke is not triggered as well, then the suction-stroke trigger flag is set and the routine ends, otherwise, the pressure sensing value obtained from the pressure sensor 103 is checked. When the pressure value is above a threshold Thd1 but below another threshold Thd2, the controller switches to Mode 1, in which the compressed air is hold in the pump. If the pressure is not lower than the threshold Thd2, then the controller goes into Mode 0 to release air, while if the pressure is not higher than the threshold Th1, the controller then switches to the Mode 3 to fill air into the pump to increase air pressure. The status of the pressing stroke is checked thereafter. The routine ends if the pressing stroke is not completed, otherwise, the suction-stroke trigger flag is set and the pressing-stroke trigger flag is reset. After the changing of the trigger flag values, the time in Mode 1 is also checked in a step 203. If it is too short, then a fault is set in a step 204 before the routine ends.

In the stroke and pressure control, to avoid momentarily going into Mode 2, in changing modes from Mode 3 to Mode 0, the controller should de-energize the solenoid valve 126 first, while in switching modes back to Mode 3 from Mode 0, the controller should energize the solenoid valve 122 first. To further avoid troubles caused by Mode 2, as shown in FIG. 1 b, a three-way solenoid valve 142 together with a two-way solenoid valve 141 can be used to replace the two-way solenoid valves 122 and 126. Referring to FIG. 1 b, the inlet of the two-way solenoid valve 141 is fluidly connected to the port 101 through a passage 121, while the outlet is fluidly connected to the inlet of three-way solenoid valve 142 through a passage 143. One outlet of the three-way solenoid valve is connected to the compressed air supply, and the other one can be fluidly connected to the muffler 124 through the passage 123 to decrease air releasing noise. With the three-way solenoid valve 142 and the two-way solenoid valve 141, the controls modes are shown in the following table:

TABLE 2 Mode Status of the Status of the number valve 142 valve 141 Actions 0 Not energized Not energized Releasing air from pump 1 Not energized Energized Keeping air in pump 2 Energized Energized Keeping air in pump 3 Energized Not Energized Filling air to pump

According to Table 2, in the system depicted in FIG. 1 b, there is no leaking mode in which compressed air supply is directly connected to ambient. Also, different from that listed in Table 1, the Mode 2 in Table 2 has all the solenoid valves 141 and 142 energized rather than just one energized, while Mode 3 has just the solenoid valve 142 energized rather than both of them energized.

In the stroke control, two events, a refill event and a pump full event, can be used to switch strokes. A refill event is triggered when a low liquid level in the pump is detected or the calculated liquid volume is low. To detect liquid level in the pump, a level sensor can be further installed inside the pump (not shown in FIG. 1), while the liquid volume can be calculated using the liquid releasing time and the pump driving pressure.

Two methods can be used in calculating the liquid volume in the pump in a pressing stroke. One is calculating the amount of liquid being released from the pump. Under the driving pressure inside the pump, when liquid starts to flow out of the pump, the flow rate of liquid through the port 102 is a function of the driving pressure. If the driving pressure is controlled constant, the flow rate is a constant value. Therefore, when the driving pressure is controlled within a narrow range, the lost liquid volume in a pressing stroke is approximately proportional to the liquid releasing time, and the liquid volume thus can be calculated by using the following equation:

V=V ₀ −K*t   (1)

, where V is the current liquid volume inside the pump; V₀ is the liquid volume when a pressing stroke starts; K is a constant, and t is the liquid releasing time. To more accurately calculate the current volume, liquid releasing rate, which is proportional to the square root of the driving pressure, can be used in the calculation:

V=V ₀∫_(t) ₀ ^(t) C√{square root over (P)}dt   (2)

where C is a constant; P is the driving pressure at moment t, and t₀ is the time moment when a pressuring stroke starts. When the flow through port 102 is further controlled by a solenoid valve (not shown in FIG. 1), the liquid releasing time is the open time of the solenoid valve in a pressing stroke. In this situation, in the equations (1) and (2), V₀ and t₀ are, respectively, the liquid volume and the time moment when the solenoid valve starts to open.

The other method is using the ratio of pressure change in Mode 1 to the amount of liquid released during the pressure change. According to the idea gas law, in Mode 1, since the air is trapped in the pump, if the effect of liquid pressure in the pump is negligible and temperature is constant, then we have the following relation:

$\begin{matrix} {{\frac{V}{P} = {{- \left( {V_{t} - V} \right)}/P}},} & (3) \end{matrix}$

where dP is the change in the driving pressure; dV is the volume change when driving pressure changes dP, and V_(t) is the tank volume. The liquid volume then can be calculated using the following equation:

$\begin{matrix} {V = {V_{t} + {P{\frac{V}{P}.}}}} & (4) \end{matrix}$

According to equation (4), unlike the first method, this method doesn't require the liquid volume at the beginning of a pressing stroke, V₀, in calculation.

The pump full event can be triggered using the pressure sensing value. In a suction stroke, when pressure inside the pump is released, the gauge pressure obtained by the sensor 103 is a proportional to the liquid level in the pump. Accordingly, the liquid level can be calculated using the pressure sensing value through the following equation:

L=Kp*P   (5)

where Kp is a constant. When the calculated liquid level is above a threshold, a pump full event is triggered.

In addition to the pressure sensing value, the refill time is also an indication to the liquid volume in the pump. The liquid refill flow-rate to the pump 100 is a function of the difference between the pressure at the port 131 and that at the port 105. Therefore, in a system of FIG. 1 a and FIG. 1 b, the liquid refill flow-rate, r_(f), is a function of the level of the liquid 133 in the tank 130, L_(t):

r _(f) =f(L _(t))   (6)

. In a control algorithm, the function in equation (6) can be realized using a lookup table, the values in which are populated with testing data. According to equation (6), the liquid volume in the pump then follows the equation below:

V=∫ ₀ ^(t) ^(f) f(L _(t))dt   (7)

, where t_(f) is the refill time. A pump full event can be triggered when the calculated liquid volume is higher than a threshold.

Since air density is much lower than liquid density, if the liquid tank 130 is empty, then in a suction stroke, the pressure sensing value is low. Therefore, pump full event will not be triggered with the pressure sensing method. By detecting a failed pump full event, we then can detect an empty liquid tank. Steps 201 and 202 in FIG. 2 show this detection.

Also, if the liquid tank 130 is empty, then air will be released from the pump rather than liquid due to the interrupt of liquid supply. It is hard to establish the driving pressure due to high volumetric releasing rate caused by low air density. Accordingly, by detecting the time of Mode 1 in a pressing stroke, we can detect an empty liquid tank or a leak pump. Steps 203 and 204 in FIG. 2 show this detection.

The pump depicted in FIG. 1 can only generate a driving pressure lower than the compressed air pressure. When higher driving pressure is required, a piston can be used to multiply the compressed air pressure. Referring to FIG. 3, inside a pump housing 300, a piston 302 has a large diameter surface 303 contacting compressed air. The other side of the piston 302 has a small diameter surface 304 contacting liquid. The piston 302 divides the pump housing 300 into three spaces: compressed air space 340, middle space 310 and liquid space 330. The compressed air space 340 is sealed from the middle space 310 with an o-ring 301 on the piston 302, while the liquid space 330 is sealed from the middle space 310 by a seal 321 in bore 320. A spring 305 is used to support the piston 302. When a pressure Pc is applied in the compressed air space 340, with the force delivered by the piston 302, the driving pressure obtained in the liquid space 330 is Pl, and

Pl=(Pc*A303−ks*x−f0)/A304   (8)

where A303 is the area of large diameter surface 303, ks is the spring constant of spring 305, x is the distance from natural position of the piston 302 to the current position, f0 is the friction force plus the static spring force, and A304 is the area of small diameter surface 304. According to equation (8), if spring force and friction force is small compared to that applied by the compressed air on the surface 303, the ratio between the areas A303 and A304, A303/A304, determines the driving pressure.

In a pressing stroke, when compressed air establishes pressure in the space 340, the piston goes downward under the pressure, pressing the spring 305 and generating driving pressure in the space 330. In a suction stroke, when the compressed air is released, the piston goes upward under the force provided by the spring 305. Thereby liquid is pulled in the space 330. Compared to the pump 100 shown in FIG. 1 a and FIG. 1 b, in the pump of FIG. 3, the suction stroke has a forced suction process. Gravity or a pressure in the liquid tank is not required in pushing the liquid into the pump.

The controls for the pump of FIG. 3 are the same as that for the pump of FIG. 1. However, the driving pressure control range is different. For a pump of FIG. 1, the driving pressure control range is from the block pressure of the check valve 104, Pb104, to the compressed air pressure Pc, while that of FIG. 3 according to equation 3 is from Pb104 to Pl(Lm),

Pl(Lm)=(Pc*A301−ks*Lm−f0)/A304   (9)

where Lm is the max. displacement of the spring 305 in the pump 300. Compared to the air-driven pumps in the previous arts (e.g. in U.S. Pat. Nos. 6,386,841, 4,645,431, and 3,963,383), in the present invention, in addition to driving pressure being controlled in closed loop, with the same compressed supply air pressure, the driving pressure in the pump is also adjustable within a broad range. Typically, the lower end of the range is limited by the check valve block pressure, while the upper end is determined by the compressed supply air pressure and the ratio of the large diameter and small diameter of the piston. The adjustable driving pressure separates driving pressure from compressed air supply pressure and thereby enables more flexible applications of the air-driven pump.

Air-driven pumps need to be refilled with a suction stroke, during which the driving pressure drops and fluid stops flowing out from the pump. For some applications with high liquid flow rate, the pump needs to be refilled from time to time. To decrease pressure drops and provide continuous fluid flow, a hydraulic buffer can be used with the pump. Referring to FIG. 4, a hydraulic buffer 400 is fluidly connected to the port 102 of the pump 300 through a port 408 and a passage 407. The hydraulic buffer has a cylindrical body 401, inside which a piston 403 separates the space into an upper chamber 410 and a lower chamber 420. To seal the lower chamber from the upper chamber, a sealing O-ring 411 is carried in an annular groove of the piston 403. In the upper chamber 410, a spring 402 is positioned in between the inner top of the cylindrical body 401 and the top of the piston 403, and the two ends of the spring are retained in grooves. In the lower chamber 420, a retainer 409 limits the position of the piston 403 when pressure in the lower chamber is released, and a pressure sensor 406 is used to measure the pressure inside the lower chamber 420. The pressure sensor 406 is electrically connected to the pump controller 110 (FIG. 1) for controlling the driving pressure, under which the liquid flows out of the hydraulic buffer through a port 404 with a check valve 405 included.

When pressure is established in the space 330 of the pump 300, through the port 102, the check valve 104, the passage 407, and the port 408, liquid flows into the lower chamber 420 of the hydraulic buffer 400 and builds up pressure therein. The pressure inside the lower chamber 420 pushes the piston 403 moving upward and pressing the spring 402. In a pressing stroke, pressure inside the lower space 420 is controlled by the controller 110 (FIG. 1) through adjusting the air pressure in the space 340 using the pressure values obtained with the pressure sensor 406. The same controls algorithm with flow chart shown in FIG. 2 can be used for the pressure control. In a suction stoke, when pressure is released in the space 330 of the pump 300, the liquid stops flowing out from the pump 300. Under the force applied by the spring 402, the piston 403 moves downward, continuously providing a pressure for the liquid in the lower chamber 420. The pressure provided by the piston 403 and the spring 402 is lower than the controlled driving pressure and will decrease with time when more liquid flows out of the hydraulic buffer. Before the pressure inside the lower chamber 420 drops below a threshold, the suction stroke finishes and the driving pressure is built up again in the next pressing stroke.

FIG. 5 shows the timing chart of pressure change. In FIG. 5, a curve 501 shows the change of pressure downstream from an air-driven pump without hydraulic buffer, e.g. the pressure downstream from the check valve 104 of the pump 300 depicted in FIG. 3, while another curve 502 shows the pressure change downstream from an air-driven pump with a hydraulic buffer, e.g. the pressure change downstream from the check valve 405 of the hydraulic buffer 400 shown in FIG. 4. In a pump without hydraulic buffer, after a pressing stroke starts, compressed air enters the pump and a pump pressure is established. When the pump pressure is higher than a check valve threshold 503, as the curve 501 shows, the pressure downstream from the pump rises up, following the pump pressure until it reaches a steady controlled value. After a suction stroke is triggered, the pressure downstream from the pump drops with the pump pressure and drops sharply to zero when the pump pressure is lower than the check valve threshold 503. In a pump with a hydraulic buffer, as shown in the curve 504, the pressure change downstream from the hydraulic buffer in the first pressing stroke is similar to that in a pump without a hydraulic buffer, except the pressure rises upon a higher check valve threshold 504, which includes two check valve thresholds (e.g. the thresholds of check valves 104 and 405). In a suction stroke, different from that in a pump without hydraulic buffer, instead of dropping sharply, under the pressure provided by the piston and spring, the pressure downstream from the hydraulic buffer just drops (almost linearly but slower) when liquid flows out. And after another pressing stroke starts, the pressure downstream from the hydraulic buffer follows the driving pressure in the hydraulic buffer once the force provided by the piston and spring is overcome.

The pressure drop in the hydraulic buffer 400 is affected by the suction stroke time. In applications where only low pressure fluctuation is allowed, the suction stroke time has to be short or a large hydraulic buffer is required. To further decrease pressure drop and continuously control driving pressure, two air-driven pumps can be used together to have the driving pressure controlled by at least one pump during working time. As depicted in FIG. 6, two pumps 610 and 620 and a hydraulic buffer 630 work together to provide a liquid flow with controlled pressure. A two-way air-intake solenoid 601 with its outlet fluidly connected to the pump 610 has its inlet fluidly connected to a side port of a Tee connector 603, through an air passage 602. The other side port of the Tee connector 603 is fluidly connected to the inlet of another two-way air-intake solenoid 606, the outlet of which is fluidly connected to the pump 620. The center port of the Tee connector 603 is connected to a compressed air supply. In the same way, the outlets of air releasing solenoids 605 and 611 of the pumps 610 and 620 are fluidly connected together through a Tee connector 608, the center port of which can be connected to a muffler 609 to decrease air releasing noise. In the liquid path, a passage 613 fluidly connects the liquid supply port of the pump 610 to a side port of a Tee connector 614, the other side port of which is fluidly connected to the liquid supply port of the pump 620 through a passage 615. The center port of the Tee connector 614 is connected to a liquid supply. In the same way, the liquid output ports of the pump 610 and 620 are fluidly connected to the two side ports of a Tee connector 618 separately through passages 616 and 617. The center port of the Tee connector 618 is fluidly connected to the liquid supply port of a hydraulic buffer 630. A pressure sensor 619 positioned inside the hydraulic buffer 630 is electrically connected to a controller 640, which also electrically controls the solenoid valves 601, 605, 606, and 611.

Liquid flows out of the hydraulic buffer 630 under a driving pressure controlled by the controller 640. Referring to FIG. 7, in which curves 701 and 702 show the changes in control mode of pumps 610 and 620 respectively, a control starts with triggering a suction stroke in the pump 610. When the suction stoke completes, a pressing stroke starts, during which driving pressure is established in the hydraulic buffer 630 and then pressure feedback control is enabled. At the same time when the pump 610 starts the suction stroke, a suction stroke is triggered for the pump 620. When the suction stroke of the pump 620 completes and the pressure feedback control in the pump 610 goes to steady, i.e., Mode 1 (keeping air in the pump) is triggered at a moment 711, the pump 620 goes into Mode 3 for a certain time positioning for driving pressure control. The Mode 3 duration time of the pump 620 is calculated using the Mode 3 and Mode 0 time of the pump 610 before its Mode 1 is triggered and after its suction stroke completes. Once the next suction stroke for the pump 610 is triggered at a moment 712, the pump 620 enters pressure feedback control continuing controlling the driving pressure in the hydraulic buffer 630. Once the pressure control goes to Mode 1 at a moment 713, the total time in Mode 3 and Mode 0 in the control of the pump 620 starting from the moment 711 is recorded, and a Mode 3 thereafter is set at moment 714 for the pump 610 with a suction stroke triggered for the pump 620. The Mode 3 duration time for the pump 610 is calculated using the recorded total time in Mode 3 and Mode 0 for the pump 620. In this way, the pump 610 and 620 work alternately to provide uninterrupted driving pressure control.

In the pressure control, the idling pump (e.g the pump 610) starts a pressing stroke (Mode 3) after the working pump (e.g. the pump 620) enables pressure feedback control. Therefore, there is a period of overlap time in which both of the pumps are in pressing stroke, e.g. between the moments of 711 and 712. During the overlap time, the working pump is in pressure feedback control, while pressure in the other one is not controlled though the pump is pressurized. Pressurizing the idling pump is to reduce the transition time after pump control is switched. The pressure in the idling pump should be close to but lower than the threshold (e.g. the upper limit of the predetermined pressure control range) above which the working pump goes into Mode 0 releasing air, since liquid in the idling pump cannot flow into the working pump, and high pressure in the idling pump could firstly cause a peak of driving pressure in the hydraulic buffer and then a valley due to feedback control in the working pump. The air filling time (Mode 3 duration time) of the idling pump is calculated using the net air filling time of the working pump, which is a function of the total air filling time (Mode 3 duration time) and the total air releasing time (Mode 0 duration time) of the working pump before its pressure control goes steady. The net air filling time of working pump is a reference. The calculated air filling time for the idling pump should be shorter than the net air filling time.

Although the apparatus and method of the invention are described herein in relation to the preferred embodiments shown in FIGS. 1-7, certain design alternations and modifications will become apparent to those of ordinary skill in the art upon reading this disclosure in connection with the accompanying drawings. For example, the pump 100 shown in FIG. 1 can be used in the structure illustrated in FIG. 4 and FIG. 6; the pump 100 in FIG. 1 a has two ports, one dedicated for air releasing and another one for air supply; and in FIG. 4, a solenoid valve in between the ports 102 and 408 can further be used to fine control the pressure in the buffer 400. It is intended, however, that the scope of the invention be limited only by the appended claims. 

1. A hydraulically driven pump system comprising: a pump housing having a pump chamber therein and at least one driving-fluid port fluidly connected to said pump chamber; a driving-fluid flow control means fluidly connected to said driving-fluid port and a first fluid for controlling supply and release of said first fluid, wherein said driving-fluid flow control means includes at least one electrically controlled solenoid valve; a driven-fluid inlet port fluidly coupled to said pump chamber through a driven-fluid flow control means, which only allows a second fluid to flow into said pump chamber; a driven-fluid outlet port though which said second fluid flows out from said hydraulically driven pump system under a driving pressure; at least one pressure sensor for providing pressure sensing values indicative to said driving pressure; and a closed-loop controller configured to control said driving pressure within a predetermined range by electrically energizing and de-energizing said at least one electrically controlled solenoid valve in said driving-fluid flow control means according to at least said pressure sensing values obtained from said at least one pressure sensor.
 2. The hydraulically driven pump system of claim 1, further comprising: a piston reciprocable in said pump chamber separating said pump chamber into a driving fluid chamber, which is fluidly connected to said driving-fluid port, and a driven fluid chamber, which is fluidly connected to said driven-fluid inlet port, and fluidly coupled to said driven-fluid outlet port.
 3. The hydraulically driven pump system of claim 1, wherein at least one electrically controlled solenoid valve in said driving-fluid flow control means is a three-way solenoid valve.
 4. The hydraulically driven pump system of claim 1, further comprising: a stroke controller configured to switch in between a suction stroke, in which said driving-fluid flow control means is controlled to release said first fluid from said pump chamber for drawing said second fluid into said pump chamber, and a pressing stroke, in which said driving-fluid flow control means is controlled to keep said second fluid from flowing into said pump chamber.
 5. The hydraulically driven pump system of claim 4, wherein said closed-loop controller is further configured to work only when said stroke controller is operating in said pressing stroke.
 6. The hydraulically driven pump system of claim 4, further comprising: a level sensor for providing level sensing values indicative to the volume of said second fluid in said pump chamber, wherein said stroke controller is further configured to switch in between said suction stroke and said pressing stroke according to at least said level sensing values obtained from said level sensor.
 7. The hydraulically driven pump system of claim 4, wherein said stroke controller is further configured to switch from said pressing stroke to said suction stroke according to at least said pressure sensing values obtained from said at least one pressure sensor.
 8. The hydraulically driven pump system of claim 4, wherein said stroke controller is further configured to switch from said pressing stroke to said suction stroke according to at least the release time of said second fluid from said driven pump system in said pressing stroke.
 9. The hydraulically driven pump system of claim 1, further comprising: a hydraulic buffer including a buffer chamber, a buffer inlet port, which is fluidly coupled to said pump chamber through a flow control means, which only allows fluid to flow from said pump chamber to said buffer chamber, and a buffer outlet port, which is fluidly connected to said driven-fluid outlet port.
 10. The hydraulically driven pump system of claim 9, wherein said at least one pressure sensor includes a pressure sensor providing sensing values indicative to the pressure in said buffer chamber;
 11. The hydraulically driven pump system of claim 9, wherein said hydraulic buffer further includes a piston reciprocable in said buffer chamber separating said pump chamber into an upper chamber and a lower chamber fluidly connected to said buffer inlet port and said buffer outlet port.
 12. The hydraulically driven pump system of claim 11, further comprising: a spring positioned in said upper chamber.
 13. The hydraulically driven pump system of claim 11, further comprising: a stroke controller configured to switch from a suction stroke, in which said driving-fluid flow control means is controlled to release said first fluid from said pump chamber for drawing said second fluid into said pump chamber, to a pressing stroke, in which said driving-fluid flow control means is controlled to keep said second fluid from flowing into said pump chamber, before said pressure sensing values indicate that said driving pressure is lower than a predetermined threshold.
 14. A hydraulically driven pump system comprising: a first pump housing having a first pump chamber therein, and at least one first driving-fluid port fluidly connected to said first pump chamber; a second pump housing having a second pump chamber therein, and at least one second driving-fluid port fluidly connected to said second pump chamber; a first driving-fluid flow control means fluidly connected to said first driving-fluid port and a first fluid for controlling supply and release of said first fluid, wherein said first driving-fluid flow control means includes at least one electrically controlled solenoid valve; a second driving-fluid flow control means fluidly connected to said second driving-fluid port and said first fluid for controlling supply and release of said first fluid, wherein said second driving-fluid flow control means includes at least one electrically controlled solenoid valve; a driven-fluid inlet port, which is fluidly coupled to said first pump chamber through a first driven-fluid flow control means that only allows a second fluid to flow into said first pump chamber, and fluidly coupled to said second pump chamber through a second driven-fluid flow control means that only allows said second fluid to flow into said second pump chamber; a driven-fluid outlet port though which said second fluid flows out from said hydraulically driven pump system under a driving pressure; at least one pressure sensor for providing pressure sensing values indicative to said driving pressure; and a closed-loop controller configured to control said driving pressure within a predetermined range by electrically energizing and de-energizing said at least one electrically controlled solenoid valve in said first and second driving-fluid flow control means according to at least said pressure sensing values obtained from said at least one pressure sensor.
 15. The hydraulically driven pump system of claim 14, further comprising: a first driven-fluid flow control means fluidly connected to said first pump chamber and said driven-fluid outlet port for only allowing said second fluid to flow from said first pump chamber to said driven-fluid outlet port; a second driven-fluid flow control means fluidly connected to said second pump chamber and said driven-fluid outlet port for only allowing said second fluid to flow from said second pump chamber to said driven-fluid outlet port; and a pump controller configured to switch among at least a first suction mode, in which said first driving-fluid flow control means is controlled to release said first fluid from said first pump chamber for drawing said second fluid into said first pump chamber while said second driving-fluid flow control means is controlled to keep said second fluid from flowing into said second pump chamber, and a second suction mode, in which said second driving-fluid flow control means is controlled to release said first fluid from said second pump chamber for drawing said second fluid into said second pump chamber while said first driving-fluid flow control means is controlled to keep said second fluid from flowing into first second pump chamber.
 16. The hydraulically driven pump system of claim 15, wherein said pump controller is configured to operate in a third mode, in which said first driving-fluid flow control means is controlled to keep said second fluid from flowing into said first pump chamber, and said second driving-fluid flow control means is controlled to keep said second fluid from flowing into said second pump chamber.
 17. The hydraulically driven pump system of claim 15, wherein said at least one pressure sensor includes a first pressure sensor providing a first pressure sensing value indicative to the pressure inside said first pump chamber and a second pressure sensor providing a second pressure sensing value indicative to the pressure inside said second pump chamber, and said closed-loop controller is further configured to control said driving pressure by operating said second driving-fluid flow control means according to said second pressure sensing value, when said pump controller is in said first suction mode, and control said driving pressure by operating said first driving-fluid flow control means according to said first pressure sensing value, when said pump controller is in said second suction mode.
 18. The hydraulically driven pump system of claim 17, wherein said at least one pressure sensor includes a buffer pressure sensor providing sensing values indicative to the pressure in said buffer chamber.
 19. The hydraulically driven pump system of claim 14, further comprising: a hydraulic buffer including a buffer chamber, a buffer inlet port, which is fluidly connected to said first pump chamber through a first buffer flow control means that only allows said second fluid to flow from said first pump chamber to said buffer chamber, and fluidly connected to said second pump chamber through a second buffer flow control means that only allows said second fluid to flow from said second pump chamber to said buffer chamber, and a buffer outlet port fluidly connected to said driven-fluid outlet port.
 20. The hydraulically driven pump system of claim 19, wherein said hydraulic buffer further includes a piston reciprocable in said buffer chamber separating said pump chamber into an upper chamber and a lower chamber fluidly connected to said buffer inlet port and said buffer outlet port. 